There has been generally known an organic electroluminescent element (also referred to as an “organic EL element” hereinafter) having a structure in which an anode constituted by a transparent electrode, a hole transport layer, a light-emitting layer, an electron injection layer, and a cathode are stacked in that order on the surface of a transparent substrate. In the organic EL element, a voltage is applied between the anode and the cathode so that light generates in the light-emitting layer and emerges to the outside through the transparent electrode and the transparent substrate.
To provide various luminescent colors to the organic EL element, the luminescent colors of light-emitting materials having different wavelengths are combined. In particular, to provide emission of light having an important color for an illumination application, three colors, namely those in red light emission, green light emission, and blue light emission, are combined. A fluorescent material and a phosphorescent material are known as light-emitting materials. Hence, an organic EL element having a multi-unit structure in which a fluorescent unit and a phosphorescent unit are stacked has been proposed (see JP 4408382 B2 and JP 4797438 B2, for example). The organic EL element having the multi-unit structure is also so-called a tandem-type element, and has a structure in which each light-emitting unit emits light, and therefore it has an advantage to form an element configuration that is suitable for each light-emitting unit.
However, the organic EL element having the multi-unit structure needs further improvement in luminous efficiency.